Here is how you can provide help about a control in a dialog box
with the [?] question mark titlebar button. To show the question
mark titlebar button create your dialog with the WS_EX_CONTEXTHELP
style.

You can provide pop-up help in a dialog box by creating an array
consisting of pairs of DWORD's. The first value in each pair is
the identifier of the control. The second value is the context
identifier you give the topic in the help file about the control.
The array must end with a pair of zero's.

The dialog box procedure must process the WM_HELP messages. The
lParam parameter of WM_HELP contains the address of a HELPINFO
structure. The hItemHandle member of this structure identifies the
control for which the user has requested Help. You must pass this
handle to the WinHelp function along with the HELP_WM_HELP command
the name of your Help file and a pointer to the array of identifiers.

The WinHelp function will search the array for the control identifier
of the specified control and then retrieve the corresponding Help
identifier. Next the function passes the Help context identifier to
Windows Help which finds the corresponding topic and displays it in
a pop-up window next to the control.